1. Field of the Invention
This invention relates to active matrix liquid crystal displays and more particularly to liquid crystal displays with redundancy of matrix elements.
2. Description of the Related Art
Demands for liquid crystal displays (LCD) for television, video and computing equipments have greatly increased. For high image quality, the high operational reliability of the active matrix LCD is usually obtained by providing redundancy in the matrix elements of the display.
The active matrix LCD having a plurality of picture elements formed on an insulator substrate in a matrix of n rows and m columns orthogonal to one another has been known. See laid-open Japanese patent application, N 60-192369, H01L 29/78, for example. Referring to FIG. 1, each picture element in the matrix includes a display electrode 3 and a switching element such as a switching thin film transistor "TFT" 4.
More specifically, the active matrix LCD includes a plurality of address buses 1-1, 1-2, 1-3, . . . , 1-n, each address bus corresponding to and a respective one of the rows of the picture elements and a plurality of data buses 2-1, 2-2, 2-3, . . . , 2-m, orthogonal to the address buses each data bus corresponding to a respective one of the columns of the picture elements.
The display electrode 3 of each picture element is connected to the respective address and data buses through the TFT 4. Scanning signals are applied to the address buses; video signals are applied to the data buses. A gate G of each TFT 4 is connected to the respective address bus; a drain D to the respective data bus; a source S to the display electrode 3.
Such an active matrix LCD has drawbacks, resulting in low operational reliability, in that while the defective switching transistor causes the failure of only the picture element that contains such defective transistor, the presence of defects (e.g., line break) in either the address or data bus cause the failure of all of the picture elements connected to such defective address or data bus.
Attempts to cure such drawbacks have been made. See Demand de brevet d'invention, N 2582431 GO9F 3/20, for example. In such attempts, the active matrix LCD provides redundancy in the matrix elements. Referring to FIG. 2, each picture element now includes two switching elements, first and second switching TFTs 4, 5 adjacent and opposite sides of the display electrode 3.
In this configuration, the display electrode 3 is connected to two sets of the address and data buses via two switching elements. The first TFT 4 is connected between the display electrode 3 and a first set of the address and data buses (i.e., 1-1 and 2-m, respectively), and the second TFT 5 between the display electrode 3 and a second set of the address and data buses (i.e., 1-2 and 2-(m+1), respectively).
In such an active matrix LCD structure, if any one of the TFTs 4, 5 of the picture element becomes defective, it would not cause the failure of the picture element since the signals can still be supplied to the display electrode through the remaining nondefective TFT. Further, the defective TFT can be removed from the matrix by means of laser beam, mechanical treatment or chemical etching. Yet further, if any one of the two address buses adjacent the picture element becomes defective, the operation of the picture element would not fail because the scanning (or address) signal can still be supplied to the display electrode through the remaining nondefective address bus. However, the above active matrix LCD has drawbacks in that if any one of the data buses becomes defective, the operation of all of the picture elements connected to such a defective data bus would fail because there is no alternate path for the video signal which would otherwise be supplied to the display electrode through the defective data bus.
Referring to FIG. 2, suppose that a scanning signal is applied to a first address bus 1-1 and that during this time, the display electrodes 3 is connected, through the TFT 4, to a first data bus 2-m. Then, the display electrode 3 will be charged to the voltage of the video signal applied to the first data bus 2-m. Further, suppose that another scanning signal is subsequently applied to a second address bus 1-2 and that during this time, the display electrode 3 is connected, through the TFT 5, to the second data bus 2-(m+1) that is defective. Then, the display electrode 3 will be discharged to the voltage (i.e., could be floating) of the defective second data bus 2-(m+1). Consequently, since the voltage at the defective second data bus may be floating (i.e., unstable), the operation of all of the picture element connected to the defective second data bus become uncontrollable.
The data bus can become defective by bad electrical contacts between the data bus and the driving circuitry, which circuitry supplies the video signal to the data bus. Defects in the output elements of the driving circuitry can also result in the defective data bus. These defects can result in the absence of the video signal on the data bus. Since the nature of defects on the data bus can be unpredictable, when the display electrode 3 is connected to the defective data bus, the operation of all of the picture elements connected thereto could become uncontrollable, and in certain situation electrically ground the display electrode.
Further, in the above active matrix, since the second switching TFT 5 can conduct currents in two opposite directions (i.e., both forward and reverse), the display electrode 3 is allowed to not only charge but also discharge through the TFT 5. In other words, the display electrode 3 can be discharged to the voltage of the defective data bus through the TFT 5.